The term displacer refrigerator is to be understood as a Gifford McMahon, Sterling or similar refrigeration machine. Single-stage refrigeration machines of this kind have a working area with a displacer. The working area is connected in alternating fashion to a high pressure and a low pressure source of gas in such a manner that during the forced to-and-fro motion of the displacer, a thermodynamic cycle is performed. The working gas is cycled preferably through a regenerator (heat accumulator for pre-cooling of the entering gas) located within the displacer. During operation of the refrigeration machine, heat is removed at one of the two ends of the working area. With a single-stage refrigerator of this kind and with helium as the working gas, temperatures down to 10-30.degree. K. may be generated. Displacer refrigerators offer the advantage of being relatively powerful and their theoretical fundamentals are well understood. Their disadvantage is the generation of vibrations caused by the mass of the displacer as it moves to-and-fro.
Also known are refrigeration machines, the operation of which is based on the principle of the pulse tube. These machines include an area with a fixed refrigerator in which in-flowing gas is cooled by exchanging the heat with the regenerator material, as well as a pulse tube into which the working gas from the area of the regenerator flows in and out periodically at one end (cold side). Connected to the other end (warm side) of the pulse tube, preferably through a constriction, is a sealed volume. By suitably selecting this throttle, the phase relationship between mass flow and pressure variation in the area of the pulse tube may be influenced for the purpose of attaining optimum performance. Besides the above approach detailed (referred to as a "Orifice Pulse Tube" design) other types of design such as ("Double Inlet", "4-valve") also exist that modify the phase relationship. The efficiency of refrigeration machines of these kinds is limited. These designs offer an advantage however, in that they do not generate any vibrations since they do not contain any moving parts.
Through a lecture on occasion of the "Cryogenic Engineering Conference" Columbus, Ohio, in July 1995 it is known to combine a displacement refrigerator with a pulse tube refrigerator. The displacer refrigerator forms the first stage and, the pulse tube refrigerator forms the second stage of a multistage low-temperature refrigeration machine. In order to ensure that the cold end of the displacer refrigerator and the warm end of the pulse tube refrigerator are at the same temperature, a thermal link made from a rigid copper panel is provided which is linked in a thermally well conducting manner to both ends of the refrigerators. Owing to this rigid link, vibrations produced by the displacer refrigerator spread to the pulse tube refrigerator. Thus, this known type of combined refrigeration machine is not suited for cooling vibration sensitive objects.